Implement API as conversions

[Plecra]

Since the bools returned by the APIs represent the guarantee made by the str type, it could be used to make the contract clearer for users. It'd also mean users aren't forced to use unsafe code with the library.

[ArniDagur]

That's a great idea! However, I want to expose both since some people may want to check if a string is UTF-8 without converting it to string.